Rotary steerable tools are one example of drilling tools used in the oil, gas and civil engineering industries to drill bore holes. Such tools are typically located between the drill bit and the drill pipe. While a rotary steerable tool may vary in principle, it will generally comprise of a bias or steering unit which exerts a force, either internally on a flexible central shaft or externally on the borehole wall to affect a change in the steering geometry to the desired direction. In one configuration, the drill pipe is connected to a drive unit located at the surface and transmits the rotary motion of the drive unit via the rotary steerable tool to the drill bit. The rotary steerable tool comprises a flexible central shaft which is connected at its top end via the necessary connections to the drill pipe. The bottom end of the flexible shaft is similarly connected to the drill bit. The flexible shaft is supported by two bearing systems, one at either end. The upper bearing is designed to prevent bending of the shaft above it and the lower bearing is typically of the angular contact type and thus allows movement of the shaft above and below it. Between the two bearings, around the centre of the length of the flexible shaft, is a bend unit that deflects the shaft. Various mechanisms may be implemented to cause the flexible shaft to be deflected to the designated amplitude so as to cause the correct angular deflection of the shaft in the required direction. It will be apparent that the portion of the flexible shaft located below the angular contact bearing will move in the contra-direction to the portion of the flexible shaft located immediately above the bearing in the bend unit. Other rotary steerable designs exist which generate deflection by alternative methods; for example, eccentric pressure pad application.
Rotary steerable tools typically incorporate a reference stabilized housing which is de-coupled, either actively or passively, from the drill string. For example, the outer housing may be restrained from rotating with respect the drill hole walls by a reference stabilizer located along the outer housing. The stabilizer typically has three or four sets of sprung rollers or contact pads which may accommodate over-gauge hole sections. The outer stabilized housing may in fact rotate in the same sense as the drill bit, but at a very slow rate as the system progresses down the hole. The reference stabilizer is designed and operated to ensure that the ratio of drill bit to outer housing turn rate does not exceed a fixed limit.
It can therefore be appreciated that as the drill bit and rotary steerable tool progress downwards along the drilled bore hole, the trajectory of the assembly, and hence that of the borehole, can be controlled. This control is typically performed and supervised by a drilling operator at the surface or start location of the bore hole.
Typically, a conventional Measurement While Drilling (MWD) survey tool is located above the rotary steerable tool in the Bottom Hole Assembly (BHA). BHA is the term used to refer to the units components and instruments positioned at the bottom of the drill string. The BHA does not necessarily include the drilling tool itself and in the present application the term BHA is used to refer to the units components and instruments placed between the drilling tool and the drill string.
Such a MWD survey tool comprises magnetometers and inclinometers which provide the drilling operators respectively with azimuthal deviation data (from a reference, e.g. magnetic north) and inclination measurements relating to the portion of bore hole in which the MWD survey tool and the BHA are currently located. When taken together these measurements provide information concerning the trajectory of the bore hole. Typically, the distance of the MWD survey tool from the surface, i.e. the well bore path length, is derived from the length of drill pipe which has been inserted into the well bore behind the MWD survey tool. Thus, the drilling operators are provided with the attitude (azimuth direction and inclination) of the bore hole at a given bore hole length. This information can be used by the drilling operators to guide the rotary steerable drilling tool.
However, there are various problems with the accuracy and latent reaction time of such a set-up. Firstly, given that the rotary steerable tool can be more than 18 feet long, the conventional MWD survey tool is located a considerable distance from the drill bit. Thus, if the drill bit veers off the desired trajectory (for example owing to rock mechanics) the drilling operator remains unaware of this condition until the MWD survey tool reaches the point at, or beyond which the unplanned deviation occurred. At this time the drill bit has progressed considerably along the deflected trajectory. Only at this point is the drilling operator aware that corrective action may be necessary.
Secondly, as MWD survey tools are typically located within the BHA at the lower end of the drill string. While drilling is in progress, the MWD survey tool is subjected to a high degree of vibration and rotary forces. This makes it difficult to obtain accurate survey data while drilling is in progress. Thus, in typical well bore drilling set-ups, drilling is stopped from time to time in order that accurate surveys may be undertaken; normally at pipe connections.
Thirdly, the drill string is typically made up of multiple segments of drill pipe with the BHA located at the lower end. The BHA also comprises tubular components of variable cross section, diameter and length. Both the drill string and BHA are limber in nature which enables the drill string to progress along the large radius curves of the drilled bore hole.
The BHA is normally composed of larger diameter, thicker walled, components, and is less limber than the drill string. In most, but not all, drilling applications, the BHA is stabilized and is nominally held concentric to the central axis of the bore hole. The standard MWD direction tool is in turn centralized within the BHA, thus providing sensor attitude data which can be said to represent the local bore hole axis, but not necessarily that of the newly drilled hole some distance below or ahead of the MWD tool.
The inherent flexibility of the BHA, and specifically, its connection to the rotary steerable system, is a necessary design attribute enabling the steering system to operate quasi-independently of the reaction forces of the BHA above. Hence, the rotary steerable system can be used to deflect the path of the bore hole in any desired attitude and direction.
The above problems could be addressed by positioning the survey sensors on the rotary steerable tool. If the survey sensors were fixed to the rotary steerable tool the measurements provided could be directly mapped to the actual direction of the rotary steerable tool hole section. As the spatial relationship between the drill bit and the rest of the rotary steerable tool will be known, the measurements taken by these sensors can also be mapped to the actual direction of the drill bit. Thus, the problems associated with the positioning of the MWD survey tool further up the drill string may be reduced and preferably eliminated.
However, in general rotary steerable tools are constructed using magnetically permeable materials. As conventional MWD survey tools contain magnetometers, they can not function accurately within the rotary steerable tool itself. Even if non-magnetic materials were used in the construction of the rotary steerable tool, the presence of large diameter steel rotating bodies can result in induced electromagnetic forces generating variable, unstable magnetic fields which preclude the use of magnetometers.
This problem is partially resolved by the use of At Bit Inclination (ABI) sensors (accelerometers) which are located within the outer housing of the rotary steerable tool itself. Such sensors are typically within a few feet of the drill bit and can thus detect relatively quickly any undesired changes in bore hole inclination at or immediately behind the drill bit trajectory and the bore hole axis. However, this sensor configuration does not provide actual azimuthal change. For example, if the drill bit veers from the desired azimuthal trajectory, but maintains the desired inclination, the operator would not be aware of this condition until the MWD survey tool data becomes available for the relevant section of hole. Additionally, the bore hole, at drill bit depth, would have strayed further from the intended trajectory.
Thus, it can be seen that present survey tool systems do not provide an accurate means for detecting the actual direction of the drill bit. This causes problems for the drilling operator when deciding to instruct a change of direction for either pre-planned or error correction reasons. In addition, knowledge of the actual position (i.e. coordinate based reference) of the drill bit, as opposed to just its direction in space, would bring additional real-time accuracy to bore hole drilling.
Another problem with existing systems is that they do not provide the drilling operator with reference quality continuous data from the survey sensors. Generally, the inhospitable environment in which the sensors may be required to operate during the drilling process precludes the availability and recording of accurate data. Thus, reference quality data is typically only obtained when drilling is interrupted and the sensors and BHA are stationary.
In view of the above problems, the provision of automated guidance of the drill bit using closed loop control is not practical in the systems outlined above. The lack of continuous, accurate information concerning the direction of the drill bit, or reference quality positional information, means that drilling operator intervention is required in order to maintain the drill bit trajectory along the pre-planned well path.